Fibers crosslinked in substantially individualized form and various methods of making such fibers have been described in the art. As is known in the art, the term "individualized crosslinked fibers" refers to cellulosic fibers that have primarily intrafiber crosslinked bonds, that is, the crosslinked bonds are primarily between cellulose molecules in a single fiber rather than being between cellulose molecules of separate fibers. Individualized crosslinked fibers are generally regarded as being useful in product applications wherein high bulk, high absorbency or both are desirable. Absorbent structures containing individualized crosslinked fibers generally exhibit an improvement in at least one significant absorbency property relative to conventional uncrosslinked fibers. Structures incorporating these fibers also generally exhibit a high degree of bulk.
A good discussion of the prior art is disclosed in U.S. Pat. No. 5,137,537, the entire disclosure of which is incorporated herein by reference.
As can be seen from said U.S. Pat. No. 5,137,537 and its disclosure of the prior art, there have been many processes for crosslinking individualized cellulosic fibers and a wide variety of crosslinking agents have been used including formaldehyde and addition products known as n-methylol agents or n-methylolamides. Dialdehyde crosslinking agents have also been used as well as polycarboxylic acids including C.sub.2 -C.sub.9 polycarboxylic acids and citric acid in particular. Crosslinking agents are usually used with a coreactant and/or catalyst. The prior art processes, although differing in specific aspects of crosslinking agents, treated cellulosic fibers with a crosslinking agent, allowed sufficient time for the crosslinking agent to penetrate the fibers, and the fibers were then defibrated into an individualized form by a wide variety of techniques including mechanical defibration as well as various fluffing devices. The fibers were dried and cured usually in air at elevated temperatures.
Although the above-referred to processes have resulted in the production of acceptable products for some applications, nevertheless, particularly for the paper industry, a continuing need exists in order to develop crosslinking technology which has the potential of being commercially significant. In particular, it is desired to develop a crosslinking technology which: produces a relatively nit free product at high throughput and low residence time in a reliable low-maintenance system; provides high thermal efficiency and can be conducted in a closed system thereby having no direct atmospheric venting and thus providing obvious advantages with regard to health and safety. The apparatus can be easily controlled, highly reliable and requires only a few moving parts.